1. Field of the Invention
The present invention relates to an image capturing apparatus including an optical low-pass filter.
2. Description of the Related Art
or video cameras using a two-dimensional image sensor (image pickup element) such as a CCD sensor or a CMOS sensor are incapable of acquiring image information of a frequency higher than a Nyquist frequency of the image sensor and thereby generate false color and moire fringe. For this reason, such image capturing apparatuses use an optical low-pass filter in order to prevent generation of the false color and moire fringe by limiting receipt of the high-frequency-image information.
Japanese Patent Laid-Open No. 10-54960 discloses an optical low-pass filter configured to separate an incident ray into four rays, by using a horizontal-separation birefringent plate and a vertical-separation birefringent plate, to form four point images. This optical low-pass filter utilizes a parallel plate having an effect of separating polarized rays from each other; the effect is provided by an oblique optic axis of a birefringent crystal. When a (μm) represents a separation width of the point images formed by this optical low-pass filter, the filter has a spatial frequency characteristic with a first cut-off spatial frequency of 1/(2a) at which contrast first disappears. Therefore, the separation width a is set such that the cut-off spatial frequency is near the Nyquist frequency of an image sensor. Such an optical low-pass filter is currently used for many single-reflex digital cameras.
On the other hand, Japanese Patent Laid-Open No. 2006-145939 discloses an optical low-pass filter configured to vary, depending on a position on a birefringent optical element where an incident ray enters, a separation width of separated rays to cut a high spatial frequency component. Birefringent optical elements having a larger thickness provide a larger separation width, that is, birefringent optical elements having a smaller thickness provide a smaller separation width. From this characteristic, the birefringent optical element disclosed in Japanese Patent Laid-Open No. 2006-145939 changes its thickness depending on the ray entrance position to vary the separation width. In addition, the birefringent optical element disclosed in Japanese Patent Laid-Open No. 2006-145939 spreads a distribution of the separation widths of the separated rays (point images) to cut a spatial frequency component in a broader range including a high frequency component. It is predicted that, in this optical low-pass filter, a distribution of the separation widths like a Gaussian distribution will make a separation width of entire incident rays having a certain width equivalent to that of point images distributed in the Gaussian distribution.
However, when using the optical low-pass filter disclosed in Japanese Patent Laid-Open No. 10-54960, a high contrast appears at a spatial frequency approximately twice of the first cut-off spatial frequency. For this reason, lens-interchangeable digital cameras having no lens resolution limitation cannot prevent the false color and the moire fringe from being generated in a high frequency band.
On the other hand, the optical low-pass filter disclosed in Japanese Patent Laid-Open No.2006-145939 has problems caused by its configuration in which the thickness of the birefringent optical element is varied depending on the ray entrance position. First, a reduction in diameter of the incident rays, that is, an increase in an F-number causes the incident rays to enter a region of the filter having a uniform thickness, which accordingly makes the separation width uniform. In this case, the optical low-pass filter disclosed in Japanese Patent Laid-Open No.2006-145939 becomes a filter merely having different spatial frequency characteristics depending on the ray entrance position and thus may generate the false color and the moire fringe at some ray entrance positions. In order to form the spread distribution of the separation widths of the point images, it is necessary to increase the diameter of the incident rays to a certain extent.
Secondly, in order to provide a distribution to the separation widths, the birefringent optical element needs to have a thickness variation of approximately several tens of percent. When a pixel pitch of an image sensor is several to several tens of micrometers, the birefringent optical element needs to have a thickness of several hundreds of micrometers. For this reason, the thickness of several hundreds of micrometers needs to be varied within a range of several tens of percent depending on the ray entrance position. This thickness variation is, however, far larger compared with a permissible variation in depth of focus of an optical system for image capturing, the depth of focus being several tens of micrometers. Therefore, this thickness variation of the birefringent optical element significantly affects an image capturing performance of the optical system.